1. Field of the Invention
The present invention relates to an aluminum alloy piping material for automotive piping excelling in corrosion resistance and workability, which is used for pipes connecting automotive radiators and heaters or pipes connecting automotive evaporators, condensers, and compressors, and to a method of fabricating the same.
2. Description of Background Art
As a piping material used for passages connecting automotive heaters, evaporators, condensers, or compressors, a single pipe consisting of an Alxe2x80x94Mn alloy such as a JIS 3003 alloy (Japanese Patent Application Laid-open No. 63-24133), and a two-layered or three-layered clad pipe consisting of 3003 alloy as a core material and an Alxe2x80x94Zn alloy sacrificial anode material such as a 7072 alloy clad on either the inner side or outer side of the core material (Japanese Patent Application Laid-open No. 56-127767) have been used. The sacrificial anode material exhibits a sacrificial anode effect on pitting corrosion occurring in the core material in a severe environment or crevice corrosion occurring when connected to a rubber hose.
However, when the Alxe2x80x94Mn alloy single pipe is used under severe corrosive environment, pitting corrosion tends to occur. Occurrence of pitting corrosion can be prevented by using the clad pipe. However, this significantly increases costs. When these piping materials are connected to radiators, heaters, evaporators, condensers, compressors, and the like, the pipe ends of the piping materials are caused to bulge. However, the Alxe2x80x94Mn alloy single pipe exhibits inferior workability, whereby working may become difficult.
The present invention has been achieved as a result of examination of the relation between the structural properties of an Alxe2x80x94Mn alloy single pipe, such as the alloy components and the compound distribution in the alloy matrix, and properties required for automotive piping materials. Accordingly, an object of the present invention is to provide an aluminum alloy piping material for automotive piping made of an Alxe2x80x94Mn alloy single pipe which excels in corrosion resistance and workability and is produced at a low cost.
(1) In order to achieve the above object, the present invention provides an aluminum alloy piping material for automotive piping excelling in corrosion resistance and workability, comprising an aluminum alloy which comprises 0.3-1.5% of Mn, 0.01-0.20% of Fe, and 0.01-0.20% of Si, wherein the content of Cu as impurities is limited to 0.05% or less, with the balance consisting of Al and impurities, wherein, among Si compounds, Fe compounds, and Mn compounds present in the alloy""s matrix, the number of compounds with a particular diameter (equivalent circle diameter, hereinafter the same) of 0.5 xcexcm or more is 3xc3x97104 or less per mm2.
(2) In this aluminum alloy piping material for automotive piping excelling in corrosion resistance and workability, the aluminum alloy may further comprise 0.4% or less of Mg.
(3) In the above aluminum alloy piping material for automotive piping excelling in corrosion resistance and workability, the aluminum alloy may further comprise 0.01-0.2% of Zr.
(4) In the above aluminum alloy piping material for automotive piping excelling in corrosion resistance and workability, the aluminum alloy piping material refined into an O material has a tensile strength of 70-130 MPa.
(5) A method of fabricating an aluminum alloy piping material for automotive piping excelling in corrosion resistance and workability, comprising hot extruding an ingot of an aluminum alloy having a composition according to any one the above (1) to (3), cold drawing the resulting extruded pipe at a working ratio of 30% or more, and annealing the cold drawn pipe, wherein the aluminum alloy piping material after being refined into an O material has a tensile strength of 70-130 MPa.
(6) In this method of fabricating an aluminum alloy piping material for automotive piping excelling in corrosion resistance and workability, the cooling rate when casting the ingot is 10xc2x0 C./second or more.
(1) The effects of alloy components and reasons for the limitations thereof, (2) the effects of compounds in the matrix and reasons for the limitations thereof, and (3) the effects of fabrication steps and reasons for the limitations thereof in the present invention are described below.
(1) Effects of Alloy Components and Reasons for Limitations
Mn increases the strength and improves corrosion resistance (pitting corrosion resistance). The Mn content is preferably 0.3-1.5%. If the Mn content is less than 0.3%, the effect may be insufficient. If the Mn content exceeds 1.5%, a large number of Mn compound particles may be formed, whereby the corrosion resistance may decrease.
Fe decreases the grain size after drawing and annealing. If the grain size is large, surface roughening or cracks tend to occur during bending or bulge formation of the piping material. The Fe content is preferably 0.01-0.20%. If the Fe content is less than 0.01%, the effect may be insufficient. If the Fe content exceeds 0.20%, a large number of Fe compound particles may be formed, whereby the corrosion resistance may decrease. The Fe content is still more preferably 0.01-0.10%.
Si decreases the grain size after drawing and annealing, thereby preventing the occurrence of surface roughening or cracks during bending or bulge formation. Moreover, Si forms Alxe2x80x94Mnxe2x80x94Si compounds and Alxe2x80x94Mnxe2x80x94Fexe2x80x94Si compounds, thereby preventing the occurrence of penetration between tools and the material during bending or bulge formation. The Si content is preferably 0.01-0.20%. If the Si content is less than 0.01%, the effect maybe insufficient. If the Si content exceeds 0.20%, a large number of Si compound particles may be formed, whereby the corrosion resistance may decrease. The Si content is still more preferably 0.01-0.10%.
Cu dissolves in a corrosive environment and decreases the corrosion resistance by readhering to the surface by reduction. If the Cu content exceeds 0.05%, the corrosion resistance significantly decreases due to readhesion under repeated humidity-salt spray conditions. The Cu content is still more preferably 0.02% or less.
Mg increases the strength of the piping material and decreases the grain size. The Mg content is preferably 0.4% or less. If the Mg content exceeds 0.4%, extrusion capability and corrosion resistance may decrease. The Mg content is still more preferably 0.2% or less.
Zr is separately distributed in a high-concentration area and a low-concentration area along the extrusion direction. These areas are alternately layered in the direction of the thickness. The low-concentration area is preferentially corroded rather than the high-concentration area, thereby forming corrosion layers. This prevents the corrosion from proceeding in the direction of the thickness, thereby improving pitting corrosion resistance and crevice corrosion resistance of the material. The Zr content is preferably 0.01-0.2%. If the Zr content is less than 0.01%, the effect may be insufficient. If the Zr content exceeds 0.2%, giant compounds are produced during casting, whereby a sound piping material cannot be obtained.
(2) Effects of compounds in the alloy""s matrix and reasons for limitations
The aluminum alloy piping material of the present invention comprises an aluminum alloy which comprises the above components, wherein, among Si compounds, Fe compounds, and Mn compounds present in the alloy""s matrix, the number of compounds with a particle diameter (equivalent circle diameter) of 0.5 xcexcm or more is 3xc3x97104 or less per mm2. Such a compound distribution prevents the occurrence of microgalvanic corrosion between the compound particles and the matrix, thereby improving the corrosion resistance. Moreover, workability is improved due to increased elongation. The distribution of compounds with a particle diameter of 0.5 xcexcm or more is still more preferably 1xc3x97104 or less per mm2.
The aluminum alloy piping material of the present invention is refined into an O material having a tensile strength of 70-130 MPa, which is a strength suitable as an automotive piping material. Moreover, the aluminum alloy piping material is provided with improved elongation and workability, thereby enabling easy bulge formation at the pipe ends.
(3) Effects of Production Steps and Reasons for Limitations
The aluminum alloy piping material of the present invention is produced as follows. A billet of the aluminum alloy having the above composition is cast by continuous casting at a cooling rate of preferably 10xc2x0 C./sec. or more. The billet is hot extruded after homogenization or without performing homogenization to prepare an aluminum alloy extruded pipe. The resulting extruded pipe is cold drawn at a working ratio ({(cross section before workingxe2x88x92cross section after working)/(cross section before working)}xc3x97100%) of 30% or more, and then annealed. The aluminum alloy is refined into an O material by this step to obtain an aluminum alloy piping material having a tensile strength of 70-130 MPa. If the drawing working ratio is less than 30%, the grain size after annealing is increased, whereby surface roughening or cracks tend to occur during bending or bulge formation.